In general, the automatic focusing method of a video camera can be roughly classified into an active type such as an infrared ray distance measuring type or an ultrasonic type, and a passive type such as an image sensing type and an image detecting type.
In particular, the principle of the active type suing an infrared ray distance measuring procedure is shown in FIG. 1, in which infrared rays are projected to an object to obtain the reflected distance measuring signal reflected by the object.
Referring to FIG. 1, the infrared light emitted from the light source of the infrared ray (not shown) such as an infrared light emitting diode (hereinafter, referred to as infrared LED) is reflected on the object 204 through the projecting lens 201, and the reflected light is focused on the light-receiving element 203 through the light-receiving lens 202.
When L indicates the distance from the projecting lens 201 to the object 204, R indicates the distance from the projecting lens 201 to the light-receiving lens 202, F is a focal length of the light-receiving lens 202, and X is the distance from the center of the light-receiving element 203 to the beam of infrared rays entering the light-receiving element 203 through the light-receiving lens 202, the relation between them is as follows: ##EQU1## Since the distance L between the projecting lens 201 and the object 204 is inversely proportional to the distance X between the center of the light-receiving element 203 and the incident point of the reflected light, the distance L can be measured.
Here, the light-receiving element 203 is divided into two channels A and B, and the electrically equivalent circuit thereof is shown in FIG. 2. In the drawings, when the P-sided electrodes are A and B and the N-sided electrode is C with reference to the structure of the light-receiving element. I.sub.0 is a total current amount produced proportional to the light incident position V.sub.0 is a driving voltage, D1 is an ideal diode, Cj is a junction capacitance, Rsh is a shunt resistor, R1 and R2 indicate resistors from the incident point of the reflected light to the electrodes A and B respectively, S is a current source, and Re is a load resistor.
Here, the photocurrents I.sub.1 and I.sub.2 can be indicated as the following relation: ##EQU2## According to the above equation 1, the ratio of (I.sub.2 -I.sub.1)/(I.sub.2 +I.sub.1), which is a ratio of the difference to the sum of the output currents, is proportional to the distance X from the middle point of the electrodes A and B, i.e., from the middle point of the light-receiving element to the incident point of the reflected light. That is, the incident angle or incident point of the beam of reflected infrared rays, which is a distance measuring signal on the light-receiving element, is changed according to the change of the distance between the object and the projecting apparatus.
Here, when the focusing of the object is correct, the reflected light enters the center of the light-receiving element, and the two converted currents I.sub.1 and I.sub.2 are to be equivalent, so that the value of the equation 1 is zero. When the focusing is diverged, the incident position of the reflected light is focused to be diverged to one side of the light-receiving element, so that the currents I.sub.1 and I.sub.2 of both channels are different from each other, and the equation 1 has a value other than 0. Accordingly, if the AF motor is controlled to keep the linear relationship between the distance X from the center of the electrodes A and B to the incident point of the reflected light and the ratio of (I.sub.2 -I.sub.1)/(I.sub.2 +I.sub.1), which is the ratio of the difference to the addition of the output currents, the exact focusing can be achieved.
Referring to FIG. 3, after the infrared light generated from the infrared LED 1 of the light emitting part is projected to the object (not shown), the infrared light is reflected to enter the light-receiving element 2, and is converted to a photocurrent in the two channel electrodes A and B of the light-receiving element 2, and is produced as an infinitesimal current signal. After the infinitesimal current signal is converted to a voltage signal in the first and second pre-amplifiers 3 provided in each channel, the output signals of the first and second pre-amplifiers 3 are filtered and amplified in the first and second synchronous filtering amplifier 4 to be synchronized o the synchronizing signal for turning on/of the infrared LED.
In the first and second buffer amplifiers 5, after the output signals of the first and second synchronous filtering amplifier 4 are integrated, the noise component is removed and the signal level is raised to be output. At this time, when the outputs of the first and second buffer amplifiers 5 are signals A and B respectively, and A/D converter 6 receives the signals A and B, determines one of four cases (i.e., .vertline.A-B.vertline..gtoreq.Vd, A.gtoreq.B, A+B.gtoreq.Vh, A+B.gtoreq.Ve), and generates the determined result to the microcomputer 7. Here, Vd, Vh, and Vl are respectively reference voltages for deciding the width of the responding range, the determining of the focusing, and the speed control range.
In the microcomputer 7, after the traveling direction and the speed or the stopping of the motor M which moves the photographing lens according to the combination of the four signals in the A/D converter 6 are determined, the speed control signal V and direction control signals F and B are supplied to the motor driver 10, and the motor M drives the photographing lens to an optimum focus position, thereby completing the automatic focusing.
Also, the microcomputer 7 supplies the clock signal CLK for making the infrared LED 1 turn on/off and detecting the synchronization of the light-receiving signal to the first and second synchronous filtering amplifiers 4 and the infrared LED driver 9, and the clear signal CLR for clearing the charges of the integration capacitor as a signal for determining the integration interval of the signal produced in the first and second synchronous filtering amplifiers 4, or for controlling the near distance limiter 8 not to control the focus for an object in the near distance, for example within 1 m.
During position detecting, signals of channels A and B produced according to the incident position of the reflected light of the light-receiving element are combined to a control signal in the A/D converter. Driving the AF motor, requires an excessive amount of hardwares, such as pre-amplifiers, synchronous filtering amplifiers, buffer amplifiers, etc., respectively composed of two channels. There also arises another difficulty in that the A/D converter convert should be drived by low bias current and offset voltage as it is composed of an operation amplifier. To improve the problem, the amount of hardware has been increased.
Moreover, since only the levels of both channel signals produced in the light-receiving element are detected and the combination signal in the A/D converter composed of an operation amplifier controls the motor M, there arises a problem in that the focusing of the video camera can not be precisely carried out when the difference of both channel signals is small.
On the other hand, for an object disposed at a near distance or infinity for which the focusing is not needed, an additional near distance limiter 8 is installed or a stopper is provided on the driving part of the photographing lens, so that it results in a complicated constitution of the video camera.